Device for attenuating unwanted waves in an electron tube

ABSTRACT

The present invention relates to a device for attenuating unwanted waves appearing in an electron tube which includes at least two coaxial cylindrical electrodes (2, 3). The electrodes (2, 3) contribute to forming the walls of a coaxial resonator. The unwanted waves to be attentuated generate surface currents in the walls of the coaxial resonator. The attenuation device includes several electrically conducting resistive elements (10) inserted into at least one wall of the resonator and arranged so as to cut the surface currents.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a device for attenuating unwanted wavesin which can develop electron tubes having coaxial cylindricalelectrodes.

This attenuation device is particularly advantageous in a grid tube inorder to avoid the appearance of undesirable modes, especially the TE11mode.

2. Discussion of the Background

Grid tubes are generally of the tetrode or triode type. A triode isrepresented diagrammatically in FIG. 1 to which reference is made. Itincludes cylindrical electrodes mounted coaxially around an axis ofrevolution XX'. The central electrode is the cathode 1 which emitselectrons when it is heated. Around the cathode 1 is a control grid 2and then an anode 3. In a tetrode there is an extra grid termed thescreen grid placed between the control grid 2 and the anode 3.

Each inter-electrode space is associated with a resonant cavity 5 whichconsists of cylindrical walls 4 extending the electrodes and is limitedheightwise by a movable plunger 6 allowing frequency adjustment. In FIG.1, only the resonant cavity associated with the control grid 2--anode 3inter-electrode space is represented completely for the sake of clarity.

Linkage between a respective electrode and a corresponding cavity wall 4is effected by a respective cup-shaped conducting collar 7 fixed to thecorresponding electrode and possibly a conducting ring 8 which includeselastic contacts and is inserted between the respective collar 7 and thecorresponding wall 4.

Since the electrodes often have to a DC potential with respect to thecathode 1, it is advisable to insert a capacitor for HF short-circuitingand blocking of direct current between a ring 8 and a cylindrical wallfor example.

The respective collars 7 are electrically insulated from one another bycorresponding ceramic spacers 9 which also have the function ofmechanical retention of the electrodes of the tube and avacuum-tightness function. A difference of several thousand volts mayexist between the control grid 2 and the anode 3 for example.

The inter-electrode space associated with its resonant cavity 5 forms acoaxial resonant circuit normally resonating in the TEM mode. Theresonant circuit can however resonate in several other modes, some ofwhich are particularly undesirable as for example the TE11 mode. This isthe dominant mode of the TE modes in coaxial resonators and it has thelowest cutoff frequency. For a given resonator, the TM modes have muchhigher cutoff frequencies and present less of a problem. Indeed, thehigher the frequency of a guided mode, the greater are the losses whichit produces in the walls. The waves corresponding to modes whose cutofffrequency is high weaken rapidly and cannot propagate.

In a grid electron tube operating as an amplifier, a very-high-frequencysignal is injected in the region of the cavity of the input resonantcircuit situated between the cathode 1 and the control grid 2 and thesignal is extracted after amplification in the region of the cavity ofthe output circuit situated between the control grid 2 and the anode 3.

Under certain conditions, unwanted resonances in undesirable modes setup in the output resonant circuit may excite the input resonant circuitand give rise to sustained oscillations.

Several means are known for avoiding sustained oscillations. These meansconsist in increasing the losses from the output resonant circuit forthe undesirable frequencies.

A first known solution consists in coupling waveguides each terminatingin an absorber to the output resonant circuit. These waveguides arestationed radially around the resonant cavity. The dimensions of theguides are chosen so as not to influence the TEM mode. These guides havea cutoff frequency which lies between the frequency of the undesirablemodes and the range of frequencies of the amplifier. The opening ofthese guides forms, in the outer wall of the resonant cavity, seriesinductances which are traversed by the surface currents generated by theTEM mode. As a consequence the frequency range of the amplifier islowered.

Moreover, the lower the freqency of the modes to be eliminated, thegreater is the cross-section of the guides. There may be difficulties inaccommodating several guides at the periphery of the resonant cavity. Inany event, even if guides can be accommodated they considerably increasethe bulkiness of the base of the tube. It has been proposed to bend themin order to reduce the bulkiness but this solution is expensive toachieve and not truly satisfactory for reducing bulkiness.

Another known solution consists in stationing above the control grid orthe screen grid, in the bottom of the anode a cylindrical conductingstructure including several resonant circuits of thedistributed-constants RLC type tuned to the frequency of the unwantedwaves to be attenuated. The material of the structure is chosen so thatit exhibits high losses. This solution has the drawback of beingselective: the conducting structure operates only within a narrow bandsince it includes resonant circuits. Given the small amount of roomavailable in the anode, it is difficult, even impossible, to accommodateother conducting structures whose resonant circuits would be tuned toother frequencies to be eliminated.

Another known solution consists in stationing, in the output cavity,ferrites exhibiting high losses for the frequencies of the unwantedwaves. However, choosing and positioning the ferrites is difficult.Futhermore, their effectiveness is not necessarily assured in alloperating cases.

Accordingly, an object of the present invention is to remedy thesedrawbacks. To accomplish this, it proposes a device for attenuatingunwanted waves for an electron tube with coaxial electrodes which issimple to make and hence cheap, which is effective over a plurality offrequencies and which does not modify the bulkiness of the tube.

SUMMARY OF THE INVENTION

The present invention relates to a device for attenuating unwanted wavesappearing in an electron tube which includes at least two coaxialcylindrical electrodes. These two electrodes contribute to forming thewalls of a coaxial resonator. The unwanted waves to be attenuatedgenerate surface currents in the walls of the coaxial resonator.According to the invention, the device for attenuating unwanted wavesincludes electrically conducting resistive elements inserted into atleast one wall of the resonator so as to reduce the surface currentsgenerated by the unwanted waves in the wall. These resistive elementsare capable of attenuating all the undesirable modes in so far as theyare traversed by the surface currents generated by these modes.

In a preferred configuration, the resistive elements are directed alongthe generatrices of a cylindrical wall of the coaxial resonator. Thegeneratrix of a surface, such as a cylindrical wall, is the line whosemotion generates the surface. Therefore, the generatrices of acylindrical wall are lines along the surface of the wall which areparallel to the longitudinal axis of the cylinder. They do not influencethe TEM mode used in the resonator since the surface currents generatedby the TEM mode are also directed along the generatrices of the wall.

Use may be made, for example, of parallelepipedal bars which may befixed into grooves in the wall. An improvement in the effectiveness ofthe attenuation is obtained by chamfering the edge of the grooves.

It is particularly advantageous to make the resistive elements frompyrolitic graphite on account of its electrical and thermal properties.This is especially the case when the resistive elements are bombarded bythe electrons emitted by the cathode.

The invention also relates to an electron tube which includes such adevice for attenuating unwanted waves. This electron tube can belong tothe family of grid tubes or magnetrons.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the invention will emerge fromthe following description given by way of non-limiting example andillustrated by the appended figures which represent:

FIG. 1, already described, a longitudinal section through a prior artgrid electron tube of triode type;

FIG. 2a: the electric and magnetic field lines in a cross section of aprior art coaxial resonator, for the TE11 mode;

FIG. 2b: the paths of the surface currents generated by the TE11 modeover one half of the internal surface of the external wall of the priorart coaxial resonator of FIG. 2a;

FIG. 3a and FIG. 3b: longitudinal and transverse sections respectivelythrough the grid and through the anode of a tube with coaxialcylindrical electrodes and furnished with an attenuation deviceaccording to the invention;

FIGS. 4a and 4b: a detail of the resistive elements of the attenuationdevice according to the invention;

FIG. 5: a partial longitudinal section through a grid tube furnishedwith an attenuation device according to the invention;

FIG. 6: a detail of an anode of a grid tube furnished with anattenuation device according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, like reference numerals designateidentical or corresponding parts which may not be described in detailfor all figures. Scale is not preserved for the sake of clarity.

FIG. 2a represents, in transverse section, a coaxial resonator of anelectron tube with coaxial cylindrical electrodes in accordance, forexample, with that represented in FIG. 1. It is assumed that the sectionhas been cut through the active part of the anode 3 and only the controlgrid 2 is represented. The section could have been cut through theresonant cavity which extends the anode and the grid. The anode 3 formsthe outer wall of the resonator and the control grid 2 the inner wall.In this section the magnetic field lines are drawn dashed and theelectric field lines are drawn solid for the TE11 mode. Two magneticfield nodes may be seen, represented by the diametrically opposed pointss and s', in the region of the internal surface of the outer wall of theresonator. The electric fields are substantially radial in this section.

FIG. 2b represents half the circumference of the internal surface of theouter wall of the resonator of FIG. 2a. The half-circumference is takenon either side of the point s. The dashed lines represent thedistribution of the surface currents which are set up in this wall forthe TE11 mode. These currents have a component directed along the axisYY' which is normal the axis XX' of the tube and in the plane of thedeveloped wall. This amounts to saying that these currents all have acomponent tangential to the outer wall of the coaxial resonator. Thesame is true in the inner wall of the resonator.

The other undesirable modes of TE type also generate currents having atangential component in the inner and inner walls of the coaxialresonator. By contast, the currents generated by the TEM useful mode aredirected longitudinally along the axis XX' and have no tangentialcomponent.

According to the invention, electrically conducting resistive elementsare inserted into at least one of the walls of the resonator, in such away that they attenuate the surface currents created in this wall by theundesirable mode or modes. The surface currents traverse these resistiveelements and dissipate energy therein in the form of heat. The unwantedwaves giving rise to these surface currents are thus attenuated.

FIGS. 3a, 3b represent respectively a longitudinal section and atransverse section through an output resonator circuit of a grid tubefurnished with an attenuation device according to the invention. Theresistive elements are bars 10 inserted into the internal wall of theanode 3 along generatrices. These bars 10 are facing the control grid 2and consequently are bombarded by the electrons emitted by the cathode.These bars attenuate the modes which generate surface currents having atangential component while leaving the surface currents which have onlya longitudinal component unperturbed.

Referring to FIG. 4a. These bars 10 will advantageously be rightparallelepipeds and in the example described there are three of them.Grooves 11 have been made in the inner wall of the anode 3 and the barsare fixed into the grooves 11 by brazing for example. In order not toperturb the TEM mode it is preferable to limit the area of the bars 10offered for electron bombardment.

The bars 10 are made from an electrically conducting resistive material.Pyrolitic graphite is a material which is particularly beneficial in themaking of these bars. Pyrolitic graphite also called oriented graphiteis essentially a crystallized graphite obtained by thermal decompositionof a gaseous hydrocarbon at the surface of a material brought to veryhigh temperature in a controlled environment. A layer of graphite isthus deposited. Pyrolitic graphite has an electrical anisotropy which isrelated essentially to its crystallographic structure. In a direction(called the C axis) normal to the plane of deposition, its electricalresistivity is much larger than in a direction parallel to the plane ofdeposition. If the graphite of the bars is oriented in such a way thatthe plane of deposition is radial, the resistance of the bars 10 will begreater than that obtained with other orientations. The losses in thecoaxial resonator will then be higher.

Furthermore, the heat resulting from the electron bombardment of theanode 3 and from the thermal dissipation related to the surface currentswill benefit from the good thermal conductivity of the pyroliticgraphite in a direction parallel to the plane of deposition. This heatwill be easily removed from the anode. FIGS. 4a and 4b show theorientation of the layers of the pyrolitic graphite.

To further improve the attentuation of the unwanted modes, chamfers 12may be made on the sides of the grooves 11. This is illustrated by FIG.4b. The cross section of the bars 10 has not been modified as comparedwith FIG. 4a.

It is preferable to use an odd number of resistive elements so as not toimpose a preferred position for the setting up of an unwanted mode.Indeed, the surface currents generated in the walls of the resonatorhave, along the periphery, an even number of current nodes. In the outerwall, these nodes (for the TE11 mode) are stationed on generatricespassing through the points s and s' represented in FIG. 2a. If tworesistive bars only were stationed at these locations, the TE11 modecould be set up without in fact being attenuated since these resistivebars would not cut the surface currents generated by the unwanted mode.

In the example just described, the resistive elements were stationed inthe anode. It is conceivable to station the resistive elements atanother location but still in at least one of the walls of theresonator. In FIG. 5 the resistive elements are inserted into the outerwall of the resonant cavity 5. It is of course also conceivable for theresistive elements 10 to be inserted into the anode 3 but in the regionof its upper part, above the grid 2, as FIG. 6 illustrates. In all theembodiments described, each resistive element is linked with one of thewalls of the coaxial resonator since the resistive elements are inserteddirectly into the walls. In all the embodiments the electric field isradial in the region of the resistive elements.

The electron tube with coaxial cylindrical electrodes as described is agrid tube. This grid electron tube may be used in particular as atelevision amplifier or in a particle accelerator or even in anindustrial installation employing high frequencies.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:
 1. A device for attenuating unwanted waves whichgenerate surface currents in an electron tube having a plurality ofwalls, comprising:a plurality of coaxial cylindrical electrodes whereineach of said electrodes provide a respective one of said plurality ofwalls and said electrodes share an axis of rotation; a plurality ofelectrically conducting resistive elements provided in grooves which arelocated in at least one of the plurality of walls, wherein saidplurality of electrically conductive resistive elements are comprised ofpyrolitic graphite, said pyrolitic graphite comprising layers which areoriented radially with respect to the axis of rotation.
 2. A device forattenuating unwanted waves which generate surface currents in anelectron tube having a plurality of walls, comprising:a plurality ofcoaxial cylindrical electrodes wherein each of said electrodes provide arespective one of said plurality of walls and said electrodes share anaxis of rotation; a plurality of electrically conductive resistiveelements provided in grooves which are located in at least one of theplurality of walls and directed substantially parallel to the axis ofrotation to attenuate the surface currents generated by the unwantedwaves in the electron tube.
 3. The attenuation device according to claim2, characterized in that the plurality of electrically conductingresistive elements are parallelelepipedal bars.
 4. The attenuationdevice according to claim 3, characterized in that the plurality ofelectrically conducting resistive elements are inserted in a pluralityof grooves located on the at least one of the plurality of walls.
 5. Theattenuation device according to claim 3, characterized in that theplurality of electrically conducting resistive elements are comprised ofpyrolitic graphite.
 6. The attenuation device according to claim 3,wherein the number of the plurality of electrically conducting resistiveelements is odd.
 7. The attenuation device according to claim 2,characterized in that the plurality of electrically conducting resistiveelements are comprised of pyrolitic graphite.
 8. The attenuation deviceaccording to claim 7, characterized in that the pyrolitic graphitecomprises layers which are oriented radially with respect to the axis ofrotation.
 9. The attenuation device according to claim 7, wherein thenumber of the plurality of electrically conducting resistive elements isodd.
 10. The attenuation device according to claim 2, characterized inthat the at least one of the plurality of walls comprises an anode. 11.The attenuation device according to claim 2, characterized in that thenumber of the plurality of electrically conducting resistive elements isodd.
 12. The attenuation device according to claim 2, characterized inthat the plurality of electrically conducting resistive elements areinserted in a plurality of grooves located on the at least one of theplurality of walls.
 13. The attenuation device according to claim 12,characterized in that the number of the plurality of electricallyconducting resistive elements is odd.
 14. The attenuation deviceaccording to claim 12, characterized in that the plurality ofelectrically conducting resistive elements are brazed elements.
 15. Theattenuation device according to claim 14, characterized in that thenumber of the plurality of electrically conducting resistive elements isodd.
 16. The attenuation device according to claim 14, characterized inthat the plurality of grooves have a chamfered edge.
 17. The attenuationdevice according to claim 14, characterized in that the plurality ofelectrically conducting resistive elements are comprised of pyroliticgraphite.
 18. The attenuation device according to claim 12,characterized in that the plurality of electrically conducting resistiveelements are comprised of pyrolitic graphite.
 19. The attenuation deviceaccording to claim 12, characterized in that the plurality of grooveshave a chamfered edge.
 20. The attenuation device according to claim 19,characterized in that the number of the plurality of electricallyconducting resistive elements is odd.
 21. The attenuation deviceaccording to claim 19, wherein the plurality of electrically conductingresistive elements are comprised of pyrolitic graphite.